Hi,
I was looking through some of these posts and couldn't find out the correct answer. I have a kitchen faucet that I installed about 4-5 months ago that has been acting strange lately. The original faucet that came with the house never put out hot water. The new faucet I bought cured that, until recently. It is the type of faucet that when you pull the handle out to the right, you push it forward and it has hot water, and pulling it towards you gives you cold water... until recently. But then after a couple of days it goes back to normal... then it goes back. This has also been happening in one of my bathroom sinks lately but I don't notice it as much since I don't use the sink in the bathroom that much. Any ideas?

Bet there is a cross connection in one of the single lever fixtures. The bath/shower is the most likely.

Turn off the hot and cold shutoffs at each shower/bath to try to isolate the problematic one. It could also be a lav or sink fixture, but they have a lower probability.

If you turn off the shower and the sink works, you found the problem. The shower.